Based on the information provided about prehistoric art, the three true facts about the artwork, specifically the "Prehistoric red bull" (likely referring to the Altamira cave paintings), would be:
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discovered in 1879 by an amateur paleontologist in Spain - The Altamira cave paintings were indeed discovered in 1879 by Marcelino Sanz de Sautuola.
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colors are ochre, red, and black - The colors used in these prehistoric cave paintings typically include ochre, red, and black.
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cave is open to the public - The Altamira cave can be visited, though access is limited to preserve the artwork.
The other statements, "shows the domestication of animals" (as the bull depicted is more often interpreted as a wild animal rather than a domesticated one) and "dates back to 18,000 BC" (the dating is often placed to around 14,000 to 20,000 years ago, so this could be ambiguous but is likely not specifically accurate), are not true in the context.
Thus, the three selected facts are accurate.